Ladies and Gentlemen -
If you are talking
"in the field" the most uniform thing about
officers' hats was...there was no uniformity. The Andrews hat was certainly present (I believe Benteen wore one), but so was almost anything else that was available, including left-over slouch, civilian felt, and many wore straw "planter's" hats. Some would wear officer's gold and black "acorn" hatband cords, plus the stamped brass crossed sabers, some would do without. For reference, see "With Crook in the Black Hills", a photo book taken during one of the campaigns. There are grouped shots of most of his officers, mostly 2nd & 3rd Cavalry. Take your pick!
In garrison, of course, it was a different story. There the kepi was the official item of headgear for officers, though it depended again on who the post commander was and what his own policy was, absent any visiting inspectors, higher echelon commanders, etc.
I personnally wear a straw planter's hat with officers hat cord and crossed sabers pinned to the front of the crown. That is in summer. In winter, I wear a muskrat fur cap sans insignia.
Hope this is of interest...